This invention relates generally to a braking system for use principally on a large vehicle and, more particularly to an eddy-current type retarder for assisting a friction brake.
In an eddy-current type retardation device disclosed in Japanese Patent Laid-Open Publication No. 298948/1989 there is disclosed a braking system in which a ring supports a plurality of permanent magnets circumferentially spaced apart at a uniform pitch and arranged within a brake drum connected to a rotational shaft, and the polarities of the permanent magnets facing the brake drum being alternately different. Between the brake drum and the magnet ring is a non-magnetic, rotatable ring retaining pole pieces having substantially the same area as the permanent magnets and circumferentially spaced at the same pitch.
When the pole pieces are moved to positions directly adjacent to the permanent magnets, a magnetic field is directed thereby through the pole pieces to act on the brake drum. Accordingly, an eddy current flows in the brake drum, producing a braking torque. However, when the pole pieces are rotated into positions wherein each pole piece is directly adjacent to two magnets circumferentially adjacent to each other, a magnetic closed circuit is produced passing through the adjacent permanent magnets, the pole piece adjacent thereto and the magnet support ring. Consequently, the magnetic field applied to the brake drum is significantly weakened, and the braking torque received becomes extremely small.
However, in the above system a problem is presented by magnetic flux that leaks from the center portions of the permanent magnets not covered by an adjacent pole piece. That leakage flux acts on the brake drum producing an undesirable dragging torque. If the thickness of the control cylinder is increased in order to minimize magnetic leakage flux from the permanent magnets to the brake drum, the entire device increases in weight, and a spacing between the permanent magnets and the brake drum is increased. Consequently, the magnetic field which permeates the pole pieces from the permanent magnets and reaches the brake drum at the time of braking decreases and the braking torque is diminished.
The object of the present invention, therefore, is to provide an eddy current type retardation device which without increased weight substantially eliminates magnetic leakage flux during non-braking periods and generates high braking torque during braking periods.